Hitherto, cold-rolled steel sheets each having a tensile strength (TS) of 1180 MPa or more have often been used for automotive components lightly worked by, for example, roll forming. Nowadays, cold-rolled steel sheets each having a tensile strength (TS) of 1180 MPa or more for press-formed components with complex shapes, such as structural members for automobiles, are increasingly being used in order to achieve both higher collision safety of automobiles and improvement in fuel efficiency owing to a reduction in the weight of automotive bodies. Thus, there is a high demand for cold-rolled steel sheet having a tensile strength (TS) of 1180 MPa or more and having excellent workability, in particular, elongation, stretch flangeability, and bendability.
In general, an increase in the strength of steel sheets has a tendency to lead to a reduction in workability. To broaden the use of high-strength steel sheets, it is thus necessary to avoid the breakage of strengthened steel sheets at the time of press forming. In the case where a steel sheet is strengthened so as to have a tensile strength (TS) of 1180 MPa or more, a very expensive scarce element, for example, Nb, V, Cu, Ni, Cr, or Mo, is actively added in addition to C and Mn from the viewpoint of ensuring strength, in some cases.
As the related art regarding high-strength cold-rolled steel sheets having excellent workability, there are Patent Literatures 1 to 4. Patent Literatures 1 to 4 each disclose a technique for producing a high-strength cold-rolled steel sheet including a tempered martensite phase and/or a retained austenite phase in a steel microstructure by the limitation of a steel chemical composition and microstructure or by the optimization of hot-rolling conditions or annealing conditions.